THE BIOGENETIC LAW FROM THE STANDPOINT 

 OF PALEONTOLOGY. 



The interest of the paleontologist in embryology, and in 

 ontogeny in general, lies wholly in the wish to know the origin 

 and relationships of biologic groups ; a scientific interpretation 

 of ontogenic data in terms of phylogeny depends on the extent 

 of preservation of the ancestral record in individual develop- 

 ment. The broad statement has often been made that each 

 animal gives in its own development an epitome of the history of 

 its race. Because of the law of heredity, this statement would 

 be true, and the record would be complete, if nothing had inter- 

 fered with the normal course of things. But, in reality, so many 

 secondary elements are introduced in development, that authori- 

 ties are very much divided as to the value of ontogenic stages 

 as records of race history. 



There can be no doubt that students of postembryonic stages 

 have been inclined to claim too much for the law of tachygenesis, 

 while, on the other hand, students of embryology have been 

 inclined to discredit it almost entirely, and to lay little stress on 

 ontogenic stages as a recapitulation of phylogeny. The reason 

 for this disagreement is not far to seek ; it lies in the field and 

 in the methods of research of the two groups of morpholo- 

 gists. 



Types of development. — Leaving out of consideration the Pro- 

 tozoa, which come into being with the essential characteristics of 

 the adults, there are, in the Metazoa, two types of development : 

 (i) the foetal type, in which the development takes place in the 

 egg, or in the body of the parent, and the young animal comes 

 into the world in form closely resembling the adult; (2) the 

 larval type, in which the young animal comes out at an earlier 

 stage of development, and reaches maturity only after considera- 

 ble metamorphosis. 



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